Sea Surface Temperature Anomaly Analysis  

The sea surface temperature anomaly chart depicts areas that are warmer or cooler than the climatic normal for the month. Areas enclosed with an isotherm that show a negative temperature are colder than normal. The example shown in

figure 4-2-9 is a Mercator hemispheric projection that uses a 2C contour interval. Regional analyses are also available that use a 1C isotherm interval.

Significant Wave Height Analysis and Prognosis  

The significant wave height (SIG WAVE HT) analysis and prognosis charts are very useful for daily shipboard sea state forecasting, heavy weather avoidance, and Optimum Track Ship Routing. Figure 4-2-10 shows a typical low-resolution hemispheric analysis. Figure 4-2-11 shows a typical regional prognosis. The hemispheric analysis uses a 6-foot wave height contour interval starting at 12 feet; the regional analyses and forecasts use a 3-foot contour interval beginning at 3 feet.

The significant wave height charts do not specifically show wind wave heights or swell wave heights. It shows computer calculations of the significant (highest one-third) of the sea waves (waves produced by the local winds) based on the fetch and duration from the analyzed and forecast surface wind fields. It will not indicate the swell waves, which may at times be higher than the sea waves. It also does not indicate a prevailing wave

direction, although this may be inferred by comparing the SIG WAVE HT charts to the corresponding surface wind charts; the primary wave direction should be the same as the direction the wind is blowing towards. Remember, though, that wind directions are reported as the direction the wind is coming from; wave directions are reported as the direction the waves are moving to. Most of the Fleet Facsimile Broadcasts also include charts, manually produced at the Oceanography Centers, that include swell wave heights. These are the Combined Sea Height charts. The combined sea height is actually the highest wave height at the points where the significant sea waves merge with the swell wave. Where these waves meet, a higher wave is formed than either the highest sea-wave height or the highest swell-wave height alone. The combined sea height, C, is calculated from both the sea-wave height (either the reported significant sea-wave height or the computer-calculated significant sea-wave height) and the swell wave height formula

The combined sea height charts also indicate by the the prevailing wave direction with an arrow. It is important that you know the differences between the combined sea height chart and the significant wave height chart. They are not intended to show the same parameters and should rarely look exactly alike. Comparison of both types of charts for corresponding times will give you a good overall picture of the swell wave, the sea waves, and the general sea state.

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